1. Field of the Invention
The present invention relates to an exposure apparatus for fabrication of a printed circuit board, for example, for use in mounting or assembling of electronic parts.
2. Description of the Prior Art
Those known and commonly called printed circuit boards include printed wire boards (which will be referred to as PWB herein below) used in computers, television sets, stereos, etc., and flexible printed circuit boards (which will be referred to as FPC herein below) used in cameras, table calculators, UTRs, etc., but the present invention relates to an exposure of a band-shaped flexible film of FPCs, such as a film carrier tape used in a package of electronic parts in a method of TAB (Tape Automated Bonding) type. To fabricate a printed circuit board, a thin photo-resist layer such as a liquid resist and a dry film resist is mounted on a substrate, and a desired pattern is formed on the photoresist layer through a step of exposing the photo-resist to light at a wavelength effective for photo-sensitization of the photo-resist through a photo-mask having a pattern drawn thereon.
In this case, there are known methods of two types: a close-contact type and a proximity type from a difference in positional relations of the substrate with the photo-mask. The former is an exposure method in which a substrate having a photo-resist mounted thereon and a photo-mask are disposed in a close-contact relation, whereas the latter is an exposure method in which a substrate and a photo-resist are disposed at a small given distance spaced apart from each other. The magnifications of the photo-mask and the pattern to be exposed in the methods of the two types are substantially the same.
Recently, however, as in ICs and the like, a reduction in size of the pattern eve in the printed board is being demanded with increasing in precision of parts and device having a printed board incorporated therein. For example, in a printed board of FPC used in a table calculator, a clock and the like, a width of each wire in a pattern within a range of about 100 to 50 .mu.m is demanded. In an exposure of a film carrier tape required for assembling of a semiconductor device in the method of TAB type which is recently prospering, an exposure at a width of each wire, for example, about 5 .mu.m, is demanded. The exposure methods of the two types given herein as the prior art methods have problems in following respects.
First, in the prior art methods of close-contact type and proximity type employing the same magnification as described above, and with reduction in size of a printed board to be fabricated, a very small photo-mask must be also prepared in proportion to such reduction in size. This makes the fabrication of the printed board difficult and causes an increase in cost.
Secondly, in the method of such proximity type, it is required for a surface on which the photo-resist layer is mounted and the photo-mask to have the same gap at any place therebetween; i.e., the parallelism is required, but the precision of parallelism cannot be increased in any way due to a warpage of the photo-mask or the like. Consequently, the image exposed may be distorted, resulting in an obstruction to the reduction in size. In serious case, the photo-mask may be brought into contact with the photo-resist and fouled or flawed, causing a defective product as in the method of the close-contact type.
Thereupon, as in the exposure method for the manufacture of ICs, the employment of a projection method has been recently reviewed even in an exposure method for fabrication of printed boards. This is because the projection method has advantages that even if the printed board to be fabricated is reduced in size, a photo-mask can be produced at available dimensions and then subjected to an exposure in a scaling-down manner to provide an image of a desired size, and that a fear of fouling or flawing due to a distortion of the photo-mask is also eliminated.
With the projection method, however, an optical aligning operation such as adjustment of the focus of an image to the photo-mask on an exposure plane is of course required. More specifically, if the photo-mask was not fabricated with a size and a precision according to a design, or the position of an original picture is not exact, an off-focusing or misalignment in position of an image with respect to the original picture may be produced in the exposure plane and hence, it is necessary to correct such phenomenon.
Even with regard to the width of each wire exposed in the printed board, there are also boards requiring an exposure of a precision of a very small gap on the order of 5 .mu.m in a case of a film carrier tape for TAB, and an optical aligning technique of a considerably high precision although not covering the order in an exposure for LSIs and VLSIs is required.
As described above, in order to adjust the off-focusing and misalignment in position of an image to the photo-mask on the exposure plane, the exposure system of the projection type has a technical subject that a means for monitoring a situation of formation of the image on the film being exposed is required and such means must be a monitoring means having a higher precision with a reduction in width of wire exposed.
On the other hand, among FPC films, there is also a printed circuit board which is made using a band-shaped film such as a film carrier tape for TAB as an insulting substrate, by forming a circuit on the film frame by frame in a longitudinal direction and cutting the film frame by frame to provide a single FPC film board.
In exposure for fabrication of such a FPC film, a mechanism for step-feeding the band-shaped film frame by frame is of course required. In this case, what needed for the exposure system of the projection type is to project an image of a pattern of a mask to be transferred, at an exact position on the film with an exact size (magnification) without off-focusing and hence, it is necessary to conduct the above-described adjustment so that the image of the pattern of the mask may be projected at a predetermined position with an exact size (magnification) without off-focusing and to locate one frame (area to be exposed) of the film to be exposed at such predetermined position Therefore, there is a subject that after the above-described step-feeding, one frame of the film must be stopped with a required high precision at the predetermined position in which an image of the photo-mask is formed.